Nuclear reactors are well known and have been utilized for steam generation by circulation of a liquid coolant from the area of the reactor core to a heat exchanger means. The nuclear reactors have been supported by various types of steel and concrete structures. The prior art support structures have been fabricated from concrete and/or steel columns and cross braces which have been designed with various walls separating the reactor components.
These prior art support structures have been difficult to fabricate and have been modified to include seismic reinforcing elements. The seismic reinforcing elements include additional reinforcing rods within the concrete structures, auxilliary snubbers, hangers and bumpers. These elements are expensive to fabricate and install, difficult to inspect and limited in their ability to protect the reactor structures from damage due to the stresses induced by high ground level acceleration.
Prior reactor support structures result in a structural configuration that significantly amplifies seismic forces to important safety related components such as the reactor vessel.
This is due to the fact that the mass to stiffness characteristics of the support structure results in a fundamental support structure frequency which is within the range of the supported component. This results in ground seismic forces being amplified by the structure to impose large seismic loads to the supported items. There are two ways to deal with the problem of accomodating seismic forces. One way is to make the structures, systems, and components sufficiently strong so they can accomodate these loads. The alternate approach is to change the configuration so that the components do not absorb all of the loads.
In the prior art both approaches have been used. Changes in configuration have included providing flexibility in the item of interest; changing the natural frequency of the component such that its frequency of vibration does not coincide with the amplified response of the supporting structure; embedment below grade of the reactor support structure; or providing an energy absorption device.
Energy absorption devices include design approaches using snubbers, ductile joints, or seismic isolation pads.
The design of liquid metal reactor vessels and piping is more sensitive to seismic disturbances than conventional reactor concepts because of thin walled vessels and piping associated with high temperature liquid metal systems. Prior art designs have had difficulty in providing support concepts which would adequately limit seismic forces to major vessels such as the reactor vessel. Current U.S. designs for commercially (1000 Mwe or greater) sized loop LMFBR plants result in high horizontal seismic shear forces being amplified by the reactor vessel structural support system for sites which have rock type (soil shear wave velocity greater than 3500 ft/sec) sites.
Studies for the loop plant have shown that for a given plant configuration the major components needing seismic protection are subjected to less severe seismic design requirements for sites with less firm soil characteristics (soil shear wave velocities less than 2000 ft/sec) due to the beneficial effects of soil-structure interaction For harder sites with rock type of foundation (soil sheer wave velocity greater than 3500 ft/sec) alternative design methods for limiting seismic forces have been investigated. For the U.S. consideration has been given to limiting the plant location to sites with suitable soil and seismic conditions; for the French Superphenix II LMRBR pool plant, consideration is being given to use of seismic isolators for limiting seismic forces to the reactor vessel; and for the United Kingdom's commercial design pool plant (CDFR) the designers approach is to embed the nuclear reactor vessel support structure below grade up to the reactor vessel support ledge in order to reduce forces to the reactor vessel (embedment does not have a large effect on modifying vertical response).
Use of embodment or seismic isolators results in a plant design which is costly and more difficult to construct. Limitations of sites to those with the proper seismicity levels and soil conditions is also undesirable since this approach could result in significantly reducing the available number of sites.